206 Dr. Goring on Microscopes. 
paribus) are sure to be the best, because the image of such needs 
but to be little magnified to procure a given power, and it must be 
evident that the more an image is magnified the more its imper- 
fections will become sensible, for no image can be free from im- 
perfection like the object from whence it is derived. 
I shall‘ here advert to a circumstance (though rather foreign to 
my subject,) relative to the proper apertures of the common mi- 
croscopic object-glasses, which is, perhaps, not duly attended to. 
It is certain that the more their apertures are reduced (within a 
certain point,) the more fog you exclude; and in this way you 
improve the instrument,—yet if this reduction is pushed too far, 
it will prevent you from seeing a certain class of objects, even 
while the vision of others seems to be ameliorated. Thus the 
parallel lines on the dust or feathers of a butterfly’s wing can be 
just seen with an object-glass of #1, inch focus, and j4 inch aper- 
ture as nearly as it can be measured: if, however, this aperture is 
very slightly contracted, they can no longer be seen with any art 
or management of the light,—at the same time other objects will 
appear fogey and indistinct with this same aperture, especially 
if opaque, and the vision of them will be improved by diminishing 
it. I am disposed, therefore, to think that the apertures should 
be regulated by this ratio of A, inch aperture to =, inch focus *. 
* The great Sir W. Herschel! has condescended to notice this subject, 
without however determining precisely what the aperture of a microscope ob- 
ject should be, in his paper in Vol. LK XVI, p. 500, of the Transactions of the 
Royal Society.—* Investigation of the Cause of that Indistinctness of Vision , 
which has been ascribed to the smallness of the Optic Pencils.’ I think, 
however, it will be found that Sir W. had not obtained pencils of rays of such 
extreme smallness as he supposed from a calculation of what the size of the 
pencil should have been, according to the powers he obtaitied, for the power of a 
compound microscope cannot be measured in the same manner ds that of a téle- 
scope, by comparing the size of the ultimate pencil of rays after it has passed the 
eye-glass with the diameter of the object-gluss or metal. Had Sir W. actually 
measured the pencils with a dynameter instead of calculating their dimension, 
he would have found them much larger than he supposed. In fact, all we 
obtain from comparing the size of the pencil of rays which enters the eye with 
the diameter of the object-glass in a microscope, is what thé power of a tele+ 
